Endothelin-1 (also referred to as “ET-1”, “small-ET-1”, and “EDN1”) is a 21 amino acid vasoconstrictor agent that was originally isolated and characterized from culture medium of porcine aortic endothelial cells. (Yanagisawa et al. (1988) Nature 332:411-415). ET-1 is derived from a ˜200 amino acid prepro-ET-1 molecule which is cleaved by an endopeptidase to produce a 38 amino acid form called “big-ET-1” (also referred to as “pro-ET-1”). Big-ET-1 is further cleaved by endothelin converting enzyme (ECE-1) to produce the vasoactive 21 amino acid ET-1 peptide. ET-1 can activate endothelin receptors type-A (“ETaR”) and type-B (“ETbR”). Activation of ETaR or ETbR in smooth muscle cells results in vasoconstriction, and intravenous administration of ET-1 to experimental animal models causes sustained elevation of arterial pressure. In the lung, endothelin gene expression is upregulated by hypoxia, leading to a feed-forward effect on vascular smooth muscle hypertrophy, vasoconstriction, inflammation and cardiac hypertrophy, and fibrosis in pulmonary hypertension.
Consistent with the foregoing properties, endothelin-1 has been implicated in various diseases and disorders including, e.g., pulmonary hypertension, heart failure, systemic hypertension, fibrotic diseases, neurodegenerative diseases, and cancer. For example, ET-1, the product of Big-ET-1 cleavage, is thought to be involved in the pathogenesis of pulmonary fibrosis due to its ability to induce fibroblast proliferation as well as stimulating collagen metabolism (see Fonseca et al. (2011) Am J Respir Cell Mol Biol 44:1-10). Elevated endothelin-1 levels have also been linked to eye diseases (e.g., glaucoma [e.g., normal tension glaucoma, hypertensive glaucoma, open angle glaucoma]) (see Sugiyama et al. (1995) Surv Ophthalmol 39 Suppl 1:S49-56; Emre et al. (2005) Br J Ophthalmol 89:60-63; Galassi et al. (2011) Invest Opthalmol Vis Sci 52:4467-4471; Ghanem et al. (2011) Ophthalmic Res 46:98-102; Tezel et al. (1997) J. Glaucoma 6:83-89; Chauhan (2008) Can J Ophthalmol 43:356-360). Accordingly, several small molecule antagonists of endothelin receptors (ETA and ETB) have been tested or proposed for the treatment of disorders such as pulmonary hypertension, cardiovascular disorders, inflammatory diseases, kidney diseases, cancers, and Alzheimer's disease.
Anti-ET-1 antibodies are mentioned in EP0406628B1; however, the antibodies of EP0406628B1 were raised against synthetic ET-1 (i.e., small-ET-1) conjugated to bovine thyroglobulin. Since small-ET-1 was used as the immunogen, the antibodies of EP0406628B1 would not be expected to bind big-ET-1, nor are they expected to have the ability to reduce or inhibit the cleavage of big-ET-1 by endothelin converting enzyme. A therapeutic agent which blocks the cleavage of big-ET-1 into small-ET-1 would act at an earlier point in the endothelin signaling process and thereby potentially provide more robust inhibitory activity than an antibody specific for ET-1 per se. Thus, there remains a need in the art for novel inhibitors of endothelin signaling, including antibodies that specifically bind human big-ET-1 and block the formation of small-ET-1.